One known heating-medium heating unit for heating a medium to be heated uses a PTC heater that uses a positive temperature coefficient thermistor device (PTC device) as a heating element. The PTC heater has a positive temperature thermistor coefficient and thus shows an increasing value of resistance as the temperature increases, which allows the current consumption to be controlled and the increase in temperature to be slowed, and thereafter, the current consumption and the temperature of the heat generating portion reach a saturation region and are stabilized; that is, the PTC heater has a self temperature control characteristic.
The PTC heater has the characteristic that the current consumption is reduced as the temperature of the heater increases, and thereafter when the temperature reaches a saturation region of a fixed temperature, the current consumption stabilizes at a low value. The use of this characteristic provides advantages in that current consumption can be reduced and an abnormal increase in the temperature of the heat generating portion can be prevented.
Therefore, PTC heaters are used in many technical fields. Also in the field of air conditioning, as disclosed in PTL 1, for example, in a hybrid-vehicle air conditioner, a heating-medium heating unit in which the PTC heater is applied to a heating unit for heating a heating medium (here, engine coolant) to be supplied to a radiator for heating air when the engine is stopped has been proposed.
In this heating-medium heating unit, two heating-medium circulation boxes are joined to each other via an O-ring in a liquidtight manner, and a flat PTC heater is closely interposed between the two heating-medium circulation boxes. The heating-medium circulation boxes are each configured such that a plurality of box components are joined via O-rings in a liquidtight manner, and the heating-medium circulation boxes each have therein a circulation path through which engine coolant, which is a heating medium, circulates.
The heating-medium circulation boxes each have a flat radiating surface in close contact with the PTC heater, and a grooved level-difference portion is formed between the flat surface and a joining surface formed on the outer periphery of each heating-medium circulation box (box component) (see FIG. 5 in PTL 1).
This is for the purpose of preventing the O-rings from being overheated by increasing the length of the heat transmission path from the PTC heater to the foregoing O-rings to prevent the O-rings interposed between the joining surfaces from deteriorating in quality due to high heat generated from the PTC heater, which would cause liquid leaks. The level-difference portion is provided with wiring members extending from the PTC heater.